From among acid-fast bacteria, human Mycobacterium tuberculosis has been widely known. It is said that the one-third of the human population is infected with this bacterium. In addition to the human Mycobacterium tuberculosis, Mycobacterium africanum and Mycobacterium bovis have also been known to belong to the Mycobacterium tuberoculosis group. These bacteria are known as Mycobacteria having a strong pathogenicity to humans.
Against these tuberculoses, treatment is carried out using three agents, rifampicin, isoniazid, and ethambutol (or streptomycin) that are regarded as first-line agents, or using four agents such as the above three agents and pyrazinamide.
However, since the treatment of tuberculosis requires extremely long-term administration of agents, it might result in poor compliance, and the treatment often ends in failure.
Moreover, in respect of the above agents, it has been reported that: rifampicin causes hepatopathy, flu syndrome, drug allergy, and its concomitant administration with other drugs is contraindicated due to P450-associated enzyme induction; that isoniazid causes peripheral nervous system disorder and induces serious hepatopathy when used in combination with rifampicin; that ethambutol brings on failure of eyesight due to optic nerve disorder; that streptomycin brings on diminution of the hearing faculty due to the 8th cranial nerve disorder; and that pyrazinamide causes adverse reactions such a hepatopathy, gouty attack associated with increase of uric acid level, vomiting (A Clinician's Guide To Tuberculosis, Michael D. Iseman 2000 by Lippincott Williams & Wilkins, printed in the USA, ISBN 0-7817-1749-3, Tuberculosis, 2nd edition, Fumiyuki Kuze and Takahide Izumi, Igaku-Shoin Ltd., 1992).
Actually, it has been reported that cases where the standard chemotherapy could not be carried out due to the adverse reactions to these agents made up 70% (approximately 23%, 52 cases) of the total cases where administration of the agents was discontinued (the total 228 hospitalized patients who were subject to the research) (Kekkaku, Vol. 74, 77-82, 1999).
In particular, hepatotoxicity, which is induced by rifampicin, isoniazid, and ethambutol out of the 5 agents used in combination for the aforementioned first-line treatment, is known as an adverse reaction that is developed most frequently. At the same time, Mycobacterium tuberculosis resistant to antitubercular agents, multi-drug-resistant Mycobacterium tuberculosis, and the like have been increasing, and the presence of these types of Mycobacterium tuberculosis makes the treatment more difficult.
According to the investigation made by WHO (1996 to 1999), the proportion of Mycobacterium tuberculosis that is resistant to any of the existing antitubercular agents to the total types of Mycobacterium tuberculosis that have been isolated over the world reaches 19%, and it has been published that the proportion of multi-drug-resistant Mycobacterium tuberculosis is 5.1%. The number of carriers infected with such multi-drug-resistant Mycobacterium tuberculosis is estimated to be 60,000,000, and concerns are still rising that multi-drug-resistant Mycobacterium tuberculosis will increase in the future (April 2001 as a supplement to the journal Tuberculosis, the “Scientific Blueprint for TB Drug Development.”)
In addition, the major cause of death of AIDS patients is tuberculosis. It has been reported that the number of humans suffering from both tuberculosis and HIV reaches 10,1700,000 at the time of year 1997 (Global Alliance for TB drug development). Moreover, it is considered that the mixed infection of tuberculosis and HIV has an at least 30 times higher risk of developing tuberculosis than the ordinary circumstances.
Taking into consideration the aforementioned current situation, the profiles of the desired antitubercular agent is as follows: (1) an agent, which is effective even for multi-drug-resistant Mycobacterium tuberculosis, (2) an agent enabling a short-term chemotherapy, (3) an agent with fewer adverse reactions, (4) an agent showing an efficacy to latent infecting Mycobacterium tuberculosis (i.e., latent Mycobacterium tuberculosis), and (5) an orally administrable agent.
Examples of bacteria known to have a pathogenicity to humans include offending bacteria of recently increasing MAC infection (Mycobacterium avium-intracellulare complex infection) such as Mycobacterium avium and Mycobacterium intracellulare, and atypical acid-fast bacteria such as Mycobacterium kansasii, Mycobacterium marinum, Mycobacterium simiae, Mycobacterium scrofulaceum, Mycobacterium szulgai, Mycobacterium xenopi, Mycobacterium malmoense, Mycobacterium haemophilum, Mycobacterium ulcerans, Mycobacterium shimoidei, Mycobacterium fortuitum, Mycobacterium chelonae, Mycobacterium smegmatis, and Mycobacterium aurum. 
Nowadays, there are few therapeutic agents effective for these atypical acid-fast bacterial infections. Under the presence circumstances, antitubercular agents such as rifampicin, isoniazid, ethambutol, streptomycin and kanamycin, a newquinolone agent that is a therapeutic agent for common bacterial infections, macrolide antibiotics, aminoglycoside antibiotics, and tetracycline antibiotics are used in combination.
However, when compared with the treatment of common bacterial infections, the treatment of atypical acid-fast bacterial infections requires a long-term administration of agents, and there have been reported cases where the infection is changed to an intractable one, finally leading to death. To break the aforementioned current situation, the development of an agent having a stronger efficacy is desired.
For example, National Publication of International Patent Application No. 11-508270 (WO97/01562) discloses that a 6-nitro-1,2,3,4-tetrahydro[2,1-b]-imidazopyran compound has a bactericidal action in vitro to Mycobacterium tuberculosis (H37Rv strain) and multi-drug-resistant Mycobacterium tuberculosis, and that the above compound has a therapeutic effect to a tuberculosis-infected animal model when it is orally administered and thus useful as antitubercular agent.
However, the compound described in the above publication differs from the compound of the present invention in terms of the basic skeleton, and it is considered to be a compound nonsimilar to the inventive compound.
Kuppsuwamy Nagarajan et al. have reported on European Journal of Medicinal Chemistry, 1989, Vol. 24, pp. 631-633 that compounds represented by the following general formula (1):
wherein R1 represents a hydrogen atom or methyl group and —(CH2)nR2 represents a chloromethyl group, C1-C7 alkyl group, isopropoxymethyl group, 3-propenyloxy-methyl group, or unsubstituted phenoxymethyl group, and compounds represented by the same above general formula (1), wherein R1 and —(CH2)nR2 bind to each other to form a cyclopentane or cyclohexane ring (16 types of compounds in total) have a bactericidal action to Mycobacterium tuberculosis (H37Rv strain).
However, the above publication describes that only the 4 types of compounds out of the above compounds are effective when they are orally administered. It also describes that the compound having the highest activity, that is, the compound (CGI-17341) represented by the above general formula (1) wherein R1 represents a hydrogen atom and —(CH2)nR2 represents ethyl, was found to have mutagenicity, and that accordingly, the development of these series of compounds as agents were abandoned.
In addition, Dilip R. Astekar et al. have reported on Antimicrobial Agents and Chemotherapy, February 1993, pp. 183-186 about the antimicrobial profile of the above compound CGI-17341. According to the report, the compound CGI-17341 has a bactericidal action to Mycobacterium tuberculosis (H37Rv strain) and multi-drug-resistant Mycobacterium tuberculosis, but it does not have the activity to atypical acid-fast bacteria, M. avium, M. intracellulare, and M. fortuitum when it is used at 250 μg/ml or less.
Moreover, Journal of Medicinal Chemistry, 1981, Vol. 24, pp. 601-604 discloses that 6-nitro-2,3-dihydroimidazo[2,1-b]oxazole compounds have a radiosensitizing ability for hypoxic mammalian cells.